The present invention relates to an electron gun apparatus for a color cathode-ray tube, and in particular to the structure of focusing electrodes for in-line electron guns which emit three electron beams in line and which are mounted on a color cathode-ray tube of shadow mask type.
In general, an electron gun apparatus for a color cathode-ray tube includes three electron guns having a plurality of grid electrodes. Electrostatic lenses are formed between grid electrodes by applying a predetermined voltage to respective grid electrodes. This electrostatic lens focuses an electron beam flux to make it strike against the fluorescent screen of the color cathode-ray tube. In order to deflect and concentrate the electron beam fluxes emitted from three electron guns to the entire surface of the fluorescent screen, deflection yokes of saddle type or toroid type are used. This deflection yoke distorts the distribution of the horizontal deflection magnetic field into a pincushion shape and distorts the distribution of a vertical deflection magnetic field into a barrel shape. This deflection yoke simplifies the structure of a convergence device because of its self-convergence effect.
While a round shaped beam spot 2 is projected onto the central part of a screen 1 formed by a fluorescent screen as shown in FIG. 7, however, a beam spot 3 distorted in a nonround shape is obtained at the peripheral part of the screen 1, resulting in a degraded resolution at the peripheral part of the screen. Such degradation in resolution caused by the deflection distortion can be mitigated by reducing the diameter of the electron beam passing through the main electron lens of the electron gun and the deflection magnetic field. In this case, however, the gap between the cathode and the main electron lens of the electron gun is narrowed. If a method of converging the electron beam by using a prefocus lens is adopted, the multiplying factor of the main electron lens becomes large and hence the diameter of the beam spot appearing in the central part of the screen becomes large, resulting in degraded resolution of the entire screen.
Electron guns as shown in FIG. 8 have been proposed to mitigate the above described problem. In FIG. 8, a first grid 11, a second grid 12, a third grid 13 and a fourth grid 14 respectively having apertures corresponding to three electron beams emitted from three cathodes 10R, 10G and 10B are successively disposed at predetermined intervals in a direction approaching a screen (not illustrated). Each numeral representing a grid attached with a suffix R, G or B denotes an aperture functioning as a hole for passing the electron beam through it. For each of apertures 11R, 11G and 11B of the first grid 11, apertures 12R, 12G and 12B of the second grid 12, and apertures 13R, 13G and 13B of the third grid 13, a single electrostatic lens is formed to function as a prefocus lens. Between the third grid 13 and the fourth grid 14, electrostatic lenses having large apertures are formed to function as the main electron lenses. In such a structure, partition plates 13a and 13b are disposed between cylindrical portions having three apertures 13R', 13G' and 13B' in the third grid 13 forming the main electron lenses. Because of these partition plates, electrostatic lenses formed in three apertures 13R', 13G' and 13B' of the third grid 13 are so distorted as to be longer in the vertical direction as shown in FIG. 9.
In this electrostatic lens so distorted as to be longer in the vertical direction, the focusing force Fn in the horizontal direction is larger than the focusing force Fv in the vertical direction as shown in FIG. 9. As a result, the electron beam is so shaped as to be longer in the vertical direction at the central part of the screen.
When such a structure is used, the aberration of the beam spot in the vertical direction is lightened at the peripheral part of the screen. As shown in FIG. 10, therefore, a round shaped beam spot 2 can be formed at the peripheral part of the screen 1.
The structure of such an electron gun for color cathode-ray tube is disclosed in JP-A-No. 54-13769, for example.
In electron guns for color cathode-ray tube having the structure heretofore described, however, a beam spot 4 formed at the central part of the screen 1 has a nonround shape (elongated in the vertical direction). Accordingly, the resolution in the horizontal direction is degraded. As a result, the resolution of the entire screen 1 cannot be raised. In other words, the resolution at the central part of the screen is different from that at the peripheral part of the screen. Means for solving these problems have thus been demanded.